1. Field of the Invention
This invention relates generally to geophones used in seismic exploration.
2. Description of the Prior Art
FIG. 1 shows a vertical geophone 10 of conventional design. FIG. 1 is a cross section taken along the longitudinal axis 12 of the geophone 10. Geophone 10 employs a cylindrical magnet 14, cylindrical upper and lower ferrous pole pieces 16, 18, and a tubular ferrous outer housing 20, which together form a magnetic circuit.
Upper and lower pole pieces 16, 18 each have a cap-like shape so that they fit over and receive the upper and lower ends of magnet 14, respectively. The tubular portion of the upper and lower pole pieces that enclose the sides of cylindrical magnet 14 are referred to herein as the pole piece lips 80, 82. Magnet 14 and pole pieces 16, 18 are received within outer cylindrical housing 20. An upper annular air gap 22 exists between upper pole piece 16 and outer housing 20, and a lower annular air gap 24 exists between lower pole piece 18 and outer housing 20.
Lower pole piece 18 and the lower end of outer housing 20 are connected to a lower end cap 26, which is in turn connected to a stake (not shown) that is placed within the ground to couple ground vibrations to the magnet and pole pieces. Lower end cap 26 is typically formed of a dielectric plastic material. An upper end cap 28 is connected between upper pole piece 16 and the upper end of outer housing 20. Upper end cap 28 is also typically made of a dielectric plastic material.
Within the annular space formed between magnet 14 and upper and lower pole pieces 16, 18, on the one hand, and cylindrical outer housing 20 on the other, an inertial member—generally a cylindrical bobbin 30—is suspended between an upper frequency-tuned spring 32 and a lower frequency-tuned spring 34. Upper frequency spring 32 is carried by a thin dielectric wafer 52, which in turn is carried by the upper pole piece 16. Lower frequency spring 34 is carried by a contact spring 36, which in turn is carried by lower end cap 26. The frequency springs allow the magnet 14, pole pieces 16, 18, and outer housing 20 to vibrate up and down axially with respect to bobbin 30 while the bobbin remains essentially motionless and decoupled from the rest of the geophone. The frequency springs are designed and tuned to provide a desired resonant frequency.
An upper electrical coil 40 is wound about bobbin 30 in the vicinity of the upper air gap 22, and a lower electrical coil 42 is wound about bobbin 30 in the vicinity of lower air gap 24. The winding direction of upper coil 40 is opposite to the winding direction of lower coil 42. An electrical circuit is formed as follows: The upper lead 80 of upper coil 40 is connected to the outer circumference of upper frequency spring 32 by solder joint. The inner circumference of the upper frequency spring makes sliding electrical contact with a first lead 60 that passes through upper end cap 28. The inner circumference of the upper frequency spring is electrically isolated from upper pole piece 16 by thin dielectric wafer 52 that is positioned therebetween. The lower lead of upper coil 40 is connected to the upper lead of lower coil 42 by a connecting wire 62. The lower lead 82 of lower coil 42 is connected to the outer circumference of lower frequency spring 34 by solder joint. The inner circumference of lower frequency spring 34 makes sliding electrical contact with the lower surface of lower pole piece 18. Contact spring 36 forces the inner circumference of lower frequency spring 34 to abut lower pole piece 18 in opposition to the force of gravity. An electrical path is formed between lower pole piece 18 and upper pole piece 16 through abutting contact of the upper and lower pole pieces with magnet 14. Finally, upper pole piece 16 makes sliding electrical contact with a second lead 64 that passes through upper end cap 28. The first and second leads 60, 64 are connected to geophone recording circuitry through a seismic cable. The arrangement of this electrical circuit allows bobbin 30 to freely rotate within geophone 10, thus minimizing the possibility of damage from rough handling.
Geophone 10 defines a magnetic circuit as follows: A magnetic flux is created by and passes axially through magnet 14. This magnetic flux is channeled through the upper and lower pole pieces 16, 18, passes radially through upper and lower air gaps 22, 24, and then passes through outer cylindrical housing 20 to form a complete magnetic circuit. The complete magnetic circuit is illustrated via flux line 71 of FIG. 2.
In operation, a terrestrial vibration causes the magnetic circuit components, and hence the magnetic flux, to vibrate up and down relative to bobbin 30, which remains essentially inertially stationary. As the radial flux lines cut the upper and lower coils 40, 42, an electromotive force is induced in the coils according to Faraday's law. This induced voltage is measured at the first and second leads 60, 64 via the electrical circuit described above.
FIG. 2 is a cut away view in partial cross-section of a portion of prior art geophone 10, shown without bobbin 30 and coils 40, 42 for simplicity. Radial lines of magnetic flux 70 crossing air gaps 22, 24 between upper and lower pole pieces 16, 18 and outer cylinder housing 20 are illustrated. Although the radial air gap magnetic flux 70 is normal to the axis of magnet 14, the flux has a tendency to fringe across the air gaps 22, 24 at the upper and lower ends of the pole piece lips 80, 82, as depicted by the bulging flux lines 72. The effect of the fringing is to increase the cross-sectional area and thus the permeance of the high-reluctance air gap. This fringing effect creates non-linearities in the magnetic flux density within the air gap, which results harmonic distortion and a non-linear geophone response. Thus it has heretofore been a concern of the prior art to maximize the linearity of the magnetic flux density passing through upper and lower air gaps to minimize harmonic distortion induced in the geophone response. Geophone 10 of prior art is designed to maximize linearity by having a long length lp of the upper and lower pole pieces 16, 18, so that the cross-sectional area of the air gaps is increased and the fringing of the flux is lowered. In order to keep the size and weight of the geophone minimal, the pole piece lips 80, 82 are lengthened to concentrate the magnetic flux near the center of magnet 14.
Some of the magnetic flux will also leak across the air gap 25 formed between the upper and lower pole pieces 14, 16. Because this flux leakage does not pass through the upper and lower coils 22, 24, it does not contribute to signal generation, and is thus referred to as a parasitic flux leakage. This parasitic flux leakage is shown by flux lines 74 in FIG. 2. Although increasing the lip length ll of the upper and lower pole pieces increases geophone response linearity, it also has the effect of decreasing the length lg of the parasitic air gap 25. This smaller lg results in lower parasitic reluctance, greater parasitic flux leakage, and thus a concomitant reduction in geophone sensitivity.
In conducting a seismic survey, multiple geophone channels are recorded. Because geophone sensitivity is low, each geophone channel typically includes between six and twelve geophones in order to produce a required voltage signal for recording. As computing power increases, it has become more desirable to conduct high resolution surveys across large geographical areas, which necessitates that large number of geophone channels are employed in a given survey. Therefore, it is likewise desirable to increase geophone sensitivity so that a fewer number of geophones are required per channel to obtain a sufficient signal strength, thus reducing the overall capital and operational cost of the survey system.
Damping of bobbin 30 is necessary so that there will not be continual oscillation of bobbin relative to the rest of the geophone. Referring to prior art geophone 10 of FIG. 1, damping of bobbin 30 is a function of the mass and the electrical conductivity of bobbin 30 (the conductivity affects the formation of eddy currents formed in bobbin 30 by Faraday induction, which eddy currents flowing in a magnetic field result in a force being exerted on bobbin 30 that opposes the motion that created the eddy currents). There is limited ability to control the conductivity of bobbin 30, and machining tolerances prohibit fine control of the mass of bobbin 30. Once a graphic design is finalized, the mass of upper and lower coils 40, 42 is fixed. The result of these factors is an inability to tightly control the damping tolerance. It is therefore desirable to control the bobbin mass more tightly in order to more precisely control the geophone damping.
Referring to prior art vertical geophone 10 of FIG. 1, the lower lead of lower coil 42 is electrically connected to lower pole piece 16 by lower frequency spring 34. Typically, the coil lead is soldered to the outer circumference of lower frequency spring 34. The inner circumference of the lower frequency spring makes a sliding electrical contact with the lower surface of lower pole piece 16, so that lower frequency spring 34 is free to rotate with respect to the lower pole piece 16.
In order to keep lower frequency spring 34 seated against lower pole piece 16 for electrical continuity, a contact spring 36 is placed between lower end cap 26 and lower frequency spring 34, which puts an upward compressive force on the inner circumference of lower frequency spring 34. However, because lower frequency spring 34 is supported by a resilient contact spring 36, rather than a rigid, stable platform, distortion of the natural sinusoidal response to an impulse is created. Moreover, tuning the geophone frequency response by control of the lower frequency spring 34 is made more difficult because of the serial spring-spring arrangement.
Other geophone designs of prior art, such as that disclosed in U.S. Pat. No. 5,119,345 issued to Woo et al., seat the lower frequency spring directly on the lower end cap. However, these design do not employ the lower frequency spring as an electrical circuit element. For example, in the Woo '345 patent, two upper pigtail springs 40 and 42 are used to provide electrical connections between the geophone coils and the geophone case. Thus, the bobbin and coil assembly have a limited ability to rotate within the geophone housing, which can result in damage to the geophone if it is subjected to rough handling during deployment or retrieval, for example.
It is therefore desirable to have a vertical geophone arrangement in which the bobbin and coil assembly is free to rotate within the geophone case and in which the lower frequency spring that forms part of the electrical circuit is not supported by a resilient contact spring.